That blades in the dark session sounds like a blast.
It's what made Henderson as powerful as he was, he had an "in-character reason" written down that gave him access to every skill or item he could need on a whim.
Honestly, this is how playing an older character should be. You only get old by living for a while first. Hand waving that as "and then he slaved away in a dead end job for thirty years" is not a great way to build an adventurer. Even ignoring the "then why is this a PC" question (which mythos-oriented games can answer easily) it's just not how humans work. Everyone finds something to keep them going psychologically, and if it's not work it's something else.
The problem, of course, is that writing decades of backstory before the game starts is a hassle. There are ways around that by allowing the addition of backstory details in play, either as a character trait, a full-on subsystem, or just informally as most people do anyway. Informal addition is least appropriate to contain mechanical benefits but is adequate as an explanation for why you're adding a certain skill to your character. As a trait works if you want one older character who tells stories from his past about why he's good at everything, though this requires a player who himself has at least some of the wisdom of age. And doing it as a subsystem is good if you want a narrativist game emulating other forms of media by having all PCs be fully realized characters whose backstories relate to situations they find themselves in and the main plot, and come up organically.